Low-noise, switchable-gain amplifier circuits are known in the art. A first embodiment is shown in FIG. 1. The amplifier in this figure has two input amplifiers 2, 4. The emitter of each amplifier is connected to an inductor 6, 8 that compensates the input capacity (base-emitter capacity) of the corresponding bipolar transistor.
Each of these input amplifiers is connected to an amplifier 10, 12 to create a cascaded circuit. The collector of each amplifier 10, 12 is connected to a respective load, or trap circuit 14, 16, which is an LC circuit comprising a capacitor 20, 22 and an inductor 18, 24 connected in parallel.
Two output transistors 26, 28 are connected as emitter-followers and each one is connected to a respective current source 30, 32. Their bases are connected to points A and B respectively. These transistors 26, 28 thus supply a signal that is an image of the signal in the corresponding LC circuit.
In order to switch between high and low gain states, the circuit comprises means 34, 36 for short-circuiting the gain stage. The input of each amplifier 2, 4 is connected to the emitter of each respective output amplifier 26, 28. This particular circuit configuration modifies the input impedance. During operation, there is a risk of creating a phase rotation between input signals IN of amplifier 2 and NIN of amplifier 4. This type of phase rotation may cause oscillation of the apparatus while destroying its amplifying effect. The apparatus would operate as an oscillator instead of an amplifier, with or without gain.
The other approach known in the art will now be described with reference to FIG. 2. References that are the same as those in FIG. 1 relate to the same components. Compensating inductors 6, 8 are connected to a current source 34 for determining the current of the input stage. Variations in the input stage current cause variations in the capacitance of input transistors 2, 4. The capacitance of the transistors is related to biasing of the transistor in question. This variation in the capacitance of each input transistor 2, 4 results in a variation in the input impedance of the system. However, any variation in input impedance reduces adaptation of the circuit. From this perspective, the second approach is no better than the first approach.
Therefore, there is a need for an amplifying apparatus that provides a dual gain or even a multiple gain, wherein such a circuit makes it possible for the amplifier's input characteristics to be unaffected irrespective of the gain selected.